This invention relates to a fluidized bed reactor and a method of operating same and, more Particularly, to such a reactor and method in which a recycle heat exchanger is formed integrally with the steam generator.
Fluidized bed reactors, such as gasifiers, steam generators, combustors, and the like, are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The entrained particulate solids are separated externally of the bed and recycled back into the bed. The heat produced by the fluidized bed is utilized in various applications such as the generation of steam, which results in an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed reactor is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface.
Other types of fluidized bed reactors utilize a "circulating" fluidized bed. According to these processes, the fluidized bed density is well below that of a typical bubbling fluidized bed, the air velocity is greater than that of a bubbling bed or the flue gases passing through the bed entrain a substantial amount of particulate solids and are substantially saturated therewith.
Also, circulating fluidized beds are characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
However, several problems do exist in connection with these types of fluidized bed reactors, and more particularly, those of the circulating type. For example, a sealing device such as a seal pot, a syphon seal, or an "L" valve and a hot expansion joint are required between the low pressure cyclone discharge and the higher pressure furnace section of the reactor, and the transfer of the separated particulate material from the cyclone back to the fluidized bed furnace has to be done by a gravity chute or a pneumatic transport system. The addition of these components add to the cost and complexity of the system. Also in these types of reactors the particulate material recycled from the cyclone to the fluidized bed furnace has to be at a fairly precise temperature. This requires an increased furnace height or the installation of wear-prone surfaces in the upper furnace to cool the particulate material before being reinjected into the fluidized bed to the appropriate temperature. This causes the furnace exit flue gases to be cooled to the point where the efficiency of the downstream convection heat exchange surfaces suffer and extra surfaces are required since the heat recovery area requires the installation of all the reheat and superheat surfaces. Further, a hot expansion joint is required between the outlet of the cyclone and the inlet to the fluidized bed furnace which is subjected to positive pressure, a distinct disadvantage.